The building blocks of all matter are atoms, which you can imagine to be
kind of like marbles, just shrunk down to an incredibly small size. If you
were the size of New Zealand, an atom would be roughly a tenth of a
millimetre wide. Often, several of these atoms will bind together into other
microscopic particles called molecules, the water molecule for instance is
made up of one Oxygen atom, and two Hydrogen atoms, hence H2O.

In liquid water the molecules are clumped densely together and can only move
about sluggishly. However, when you heat the water the molecules start to
move faster and faster. Eventually they break free of each other and the
water turns into steam.

Heating a pot of water turns some of it into steam. Since the pot is hottest at its bottom, steam
will appear there first in the form of bubbles. If you keep heating, more
bubbles will appear as more water turns to steam, and the bubbles will get
bigger. Eventually, the bubbles will be large enough to leave the bottom of
the pot and float towards the top. If you watch the bubbles carefully,
you'll notice they actually disappear before reaching the top!

The water near the top of the pot is colder than at the bottom, and cools the bubbles
down turning them back to water. When this happens they collapse, making a
small popping noise. This is why water hisses as you heat it. Keep heating
the pot, and eventually the water will be hot enough that the bubbles reach
the top without collapsing, and the hissing sound disappears. Heat even
more, and so much steam is produced that the water boils.